Method of controlling fluid infiltration



Jan. 13, 1953 J NEUMAN 2,625,374

METHOD OF CONTROLLING FLUID INFILTRATION Filed Jan. 22, 1946 2SHEETS-SHEET l LEONARD J. Nil/HAN I N V EN TOR.

By ma M,

A TTORNE Y5 V Jail. 13,'1953 NEUMAN METHOD OF CONTROLLING FLUIDINFILTRATION 2 SHEETS-SHEET 2 Filed Jan. 22, 1946 LEONARD J. NEUMININVENTOR.

By M

ATTORNEYS Patented Jan. 13, 1953 UNITED STATES TENT OFFICE METHOD OFCONTROLLING FLUID INFILTRATION 6 Claims.

This invention relates to a method for controlling fluid infiltration,or invasion, into the geological strata penetrated by bore holes and fordeterminingpermeability of such strata. The application is acontinuation of copending application, Serial Number 329,625 filed April15, 1940, now abandoned.

A purpose in the use of various components of modern drilling muds inthe practice of drilling wells by the rotary method is that ofpreventing the invasion of permeable formations traversed by the borehole. Such muds do, for the most part, prevent or minimize invasion fromsuch pressure differentials as may exist across the filter cake formedthereby. They do not, however, prevent invasion by osmosis anddifferential diffusion through the filter cake and into the formations.

It is now commonly known that, even when muds having the best of cakeforming characteristics are usec, the formations traversed by the borehole are seriously invaded by fluid from the muds, often to the extentof thirty or forty feet from the axis of the well bore. This invasion isdecidedly detrimental inasmuch as it aiiects conventional well loggingin such a manner that the interpretation of the logs is dilficult, andoften impossible. For this reason additional laborious operations arefrequently necessary to clarify information, and in some cases apotentially productive horizon is entirely overlooked. It is also to benoted that fluid invasion of potentially productive formations oftenseriously affects production characteristics of the well.

No drilling fluids that are now available completely eliminate invasionby osmosis and difierential diffusion, and it has therefore beenimpossible heretofore to eliminate the difiiculties to which referencehas just been made.

The terms differential diffusion and osmosis as used herein includethose phenomena which cause infiltration or invasion of liquidcomponents of drilling mud into penetrated for- Another object is tocontrol differential diffusion and molecular movement in a well duringdrilling and during subsequent operations of bringing the well intoproduction whereby the potentially productive horizons may beeffectively logged and optimum production characteristics areestablished.

A further object is to prevent infiltration by osmosis and differentialdiffusion into shales so that such formations may be effectively drilledby avoiding the heaving of such shales into the well.

Another and more specific object is to apply an electrical potentialbetween the interior of a well and the surrounding formations toovercome the tendency for differential diffusion to take place throughthe filter cake.

A still further object is to maintain such electrical potential afterdrilling is completed and prior to the completion of the Well so thatdifferential diffusion during such interval is minimized.

The invention also comprehends the application, for a predeterminedperiod of time, of a reversed potential between the drilling fluid andthe surrounding formation whereby controlled infiltration or invasion iseffected, and correlation of electrical logs prior and subsequent tosuch infiltration affirmatively indicates permeability of the respectivepenetrated formations.

The foregoing are primary objects which, together with other objects andadvantages of the invention, will be more fully apparent from thefollowing description considered in connection with the accompanyingdrawings in which:

Fig. 1 is a sectional View through the earth showing an illustrativeembodiment of the invention;

Fig. 2 is a sectional View similar to that shown in Fig. 1, butillustrating the extent of controlled invasion of penetrated formationsfrom which permeability of the various formations may be readilydetermined;

Fig. 3 is a graphical illustration showing the relation for a givenformation between the volume invaded and the potential applied;

Fig. 4 shows an elemental unit solid illustrating the manner in which aquantity of fluid Q is caused to flow through an elemental volume V by apotential gradient .AE/A:c.

In the practice of the invention as illustrated in Fig. 1 of thedrawings, the bore hole I is pro vided with a surface casing 2. Belowthis casing the bore hole penetrates successive porous, somewhat porousor permeable formations 3, 4, 5 and 6 a din ly greater depths. It is tobe understood that the disclosure is illustrative only as fewer or agreater number of permeable formations may be considered.

The bore hole I is filled with or usually contains a drilling fluid ormud 1. Such drilling fluid or mud is usually made up of a water basewhich carries solid or colloidal matter which is compounded for thepurpose of accomplishing various functions during the drilling operationand is usually of such nature that a filter cake 8 is formed on thewalls of the bore. As already explained such filter cake is usuallyrelatively impermeable and capable of preventing or minimize invasion ofthe formations by liquids from the mud due to mechanical pressuredifferential at the face of the Well bore 2. It has been found that sucha filter cake on the formation face is incapable of preventing invasionof the formation by liquid from the mud due to molecular movement, fromosmosis and differential diffusion.

To prevent or minimize invasion from osmosis and differential diffusionthe invention comprehends the use of a suitable source of electricalpotential such as the generator Ill. This generator is provided with arheostat l I so that the terminal potential of the generator may bereadily controlled within desired limits. Inasmuch as the resultsobtained in accordance with the invention are primarily dependent upon apotential field at the well bore, and not the current intensity withinsuch area, the terminal voltage of the generator 10 may vary betweenrelatively wide limits. For example, in the Gulf Coast area of theUnited States where the moisture content of the earths crust is high, aterminal voltage of fifty volts is adequate. On the other hand, wheredrilling operations are carried out in more arid areas, such as theuplands of West Texas, a terminal voltage as much as two-hundred andtwenty volts may be necessary. These suggested potentials are given byway of illustration and it is not intended that the invention shall beconfined to the use of these exact potentials.

The terminals 12 and i3 of the generator H] are connected to a doublepole double throw reversing switch [4 so that the polarity suppliedtothe circuit conductors l5 and 16 may be readily controlled. Whendrilling deep wells such as oil and gas wells, the formations traversedcontain connate waters that are mineralized, that is containing varioussalts which carry a negative potential. Under these conditions and inaccordance with the invention, it is necessary that the interior of thebore hole be positive with respect to the formation penetrated. To thisend the switch [4 is closed so that the conductor I5 is connected to thepositive terminal of the generator l0 and to the drill stem within thewell bore.

The negative terminal of the generator [0 is connected to a control box20 through which connections may be selectively made to a plurality ofgrounds 21, 22, 23 and 24 arranged at selected distances from the wellthat there is a desired distribution of electrical energy as indicatedby the lines 25, 2B, 21 and 28 respectively.

Since two to ten weeks of time is usually required to complete a well,the amount of time during which invasion may take place during suchoperations as changing of bits, logging, running pipe, etc., isrelatively small and for this reason during such time, while a potentialis not applied in accordance with the foregoing, is negligible."Following the actual drilling operations in which this invention isutilized in the manner above indicated there is a period during whichother operations such as logging. setting of casing, testing forproduction and cleaning of the well are carried out. During this periodit is desirable that potential conditions as above, be maintainedinsofar as practicable to prevent molecular movement or migration. Tothis end any suitable electrode may be utilized'in the well. Forexample, a 'cable may be lowered into the well for temporary use as anelectrode or any tubing string lowered into the well will be so used.

After drilling is completed and before the casing is set, the well islogged. One phase of the invention comprises the carrying out of thesubsequent step of effecting controlled invasion so that permeability ofpenetrated formations may be determined. For this purpose the switch I4is thrownso that a negative potential is applied to an electrode in thewell for a predetermined length of time as illustrated in Fig. 2 wherebyinfiltration into the various formations 3, 4, 5 and 6 is produced asindicated at 3|, 32, 33 and 34 respectively. It is to be noted thatinvasion is a function of permeability and that therefore the extent ofinvasion varies in the general manner indicated.

At the end of time, t, the well is relogged for the purpose ofdetermining the depth of the controlled invasion of each formation asresulting from the application of a potential, E. These measurementsprovide values of V, the invaded volume, corresponding to the appliedpotential E existing during the time, t. These data enable the plottingof the curve 35 shown in Fig. 3. The slope of this curve is:

AV/AE=tan (A) But, AV=KAx which is a function of the depth of theinvasion .r. One may therefore write:

The quantity of fluid, Q, entering a unit cube having volume V duringtime, t, as illustrated in Fig. 4 is related to AE/Aa: in the followingmanner:

In this equation P is permeability of the formation under consideration.Now from equations (B) and (C) P Q/tzKz tan (D) :KaV tan (E) Inequations (D) and (E1) V and tan are determined in the manner justindicated.

It seems apparent that the method gives the average permeability of alarge sample of formation in situ and is therefore extremely advantageous over the ordinary method of determining permeability from smallcontaminated samples obtained by coring and the like.

The practice of the invention as above set forth pertains to operationscarried out where connate waters are mineralized. The invention is alsoapplicable where drilling operations'are carried out for the purpose ofproducing fresh water. In such case, however, technique is modified tothe extent that potentials, as above set forth, are reversed inpolarity. This is necessary because under these conditions the drillingmud is made more highly mineralized than are the fresh waters of theformations through which drilling is carried out.

Broadly the invention comprehends a method for controlling fluidinfiltration. or invasion into geological strata penetrated by boreholes and also for determining permeability of such strata.

The invention claimed is:

1. The method of preventing infiltration, by differential diffusion, offluids from drilling mud into the formations containing mineralizedwater penetrated by a well bore and determining permeability of thepenetrated formations comprising, applying a positive electricalpotential between the drill string and the earth surrounding the wellbore, maintaining said potential during drilling, electrically loggingthe bore, applying a reversed potential between the mud and thesurrounding formation whereby controlled infiltration is effected, andthereafter re-logging the well, whereby the rate of infiltration andhence permeability of the penetrated formations are determinable.

2. In the drilling of wells by the rotary method wherein drilling fluidis pumped downwardly through a drill string during drilling, the methodof preventing infiltration by osmosis of the drilling fluid intopenetrated formations containing mineralized water during drilling, anddetermining permeability of the formations comprising, applying apositive electrical potential between the drilling fluid and the earthsurrounding the well bore and maintaining said potential, duringdrilling, at a sufficiently high potential with respect to thesurrounding formation to overcome the tendency for infiltration byosmosis, electrically logging the bore so formed, applying a reversedpotential between the drilling fluid and the surrounding formations fora predetermined time, and thereafter re-logging the well to determinethe amount of infiltration during such time.

3. The method of preventing infiltration, by differential diffusion, offluids from drilling mud into the formations, containing fresh water,penetrated by a well bore and determining permeability of the penetratedformations comprising, applying a negative electrical potential betweenthe drill string and the earth surrounding the well bore, maintainingsaid potential during drilling, electrically logging the bore, applyinga reversed potential between the mud and the surrounding formationwhereby controlled infiltration is effected, and thereafter re-loggingthe well, whereby the rate of infiltration and hence permeability of thepenetrated formations are determinable.

4. In the drilling of wells by the rotary method wherein drilling fluidis pumped downwardly through a drill string during drilling, the methodof preventing infiltration by differential diffusion of the drillingfluid into penetrated formations containing fresh water during drilling,and determining permeability of the formations comprising, applying anegative electrical potential between the drilling fluid and the earthsurrounding the well bore and maintaining said potential, duringdrilling, at a sufficiently high potential with respect to thesurrounding formation to overcome the tendency for infiltration bydifferential diffusion, electrically logging the bore so formed,applying a reversed potential between the drilling fluid and thesurrounding formations for a predetermined time, and thereafterrelogging the well to determine the amount of infiltration during suchtime.

5. The method of determining the permeability of a porous earthformation penetrated b drilling a well bore while a drilling mud cake iscovering said earth formation comprising, forming a drilling mud cake ona porous earth formation strata, said cake minimizing invasion of wellfluids from said well bore into said porous earth formation strata bypressure differentials but permitting an invasion of said well fluids byosmosis, thereafter applying a first potential difference between thewell fluids and the earth surrounding the well bore for a predeterminedperiod of time to control said invasion by said osmosis, thereafterelectrically logging the bore to record the extent of invasion of saidwell fluid, applying a second potential difference difierent from' saidfirst potential difference for the same period of time as said firstpotential difference was applied, and thereafter electrically loggin theextent of invasion of said well fluid during application of said secondpotential difference, the two loggings enabling a mathematicalcomputation of the permeability of said porous earth formation in situ.

6. The method of determining the permeability of a porous earthformation penetrated by drilling a well bore in situ comprising, forminga drilling mud cake on said porous earth formation, while said cakeremains thereon applying a first electrical potential difference betweenthe well fluids and the earth surrounding the well bore to controlosmotic invasion of said porous earth formation through said mud cake,electrically logging the bore to record the extent of said invasion,thereafter applyin additional electrical potential differences betweensaid well fluid and said earth surrounding the well bore, saidadditional potential differences being of different voltage than saidfirst potential difference and each of the other of said additionalpotential differences, and electrically logging the well liquid invasioncaused by each potential difference to thereby obtain data fordetermining the permeability of said porous earth formation in situ.

LEONARD J. NEUMAN.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,084,143 Hummel June 15, 19372,172,625 Schlumberger Sept. 12, 1939 2,211,696 Irons Aug. 13, 19402,212,274 Martienssen Aug. 20, 1940 2,217,857 Byck Oct. 15, 19402,283,206 Hayward May 19, 1942

